1. Field of the Invention
This invention relates to heating elements such as thermal printing heads, heaters and the like, and more particularly to heating resistors used in the heating elements. The invention also relates to a method for making such heating resistors.
2. Description of the Prior Art
Heating resistors made of a variety of materials have been hitherto proposed for use in heaters or thermosensitive heads. In recent years, attention has been particularly drawn to the application of such a heating resistor to a thin film thermal printing head. The thin film thermal printing head (hereinafter referred to simply as thermal printing head) is used to cause a thermosensitive paper to be developed in color in the form of desired mosaic patterns of dots so as to print pictures, letters or characters on the paper according to information signals. For this purpose, a number of structures for the thermal printing head have been proposed as described, for example, in "Kinzoku Hyomen Gijitsu (Journal of Metal Finishing society of Japan)" 34, (6) (1983, P. 271-277).
According to the literature cited above, a heating resistor used for the above purpose is predominantly made of tantalum nitride (Ta.sub.2 N) in which tantalum serves as a conductive material and nitrogen is used as an electrically insulating material. The use of tantalum nitride (Ta.sub.2 N) in a known heating resistor is described with reference to FIGS. 5, 6a and 6b.
FIG. 5 is a schematic sectional view showing an essential part of a thermal printing head using a conventional tantalum nitride (Ta.sub.2 N) heating resistor. In the figure, only one heating resistor mounted on an insulating substrate is sectionally shown although a thermal printing head has a great number of heating resistors therein. The thermal printing head includes an insulating substrate 11 made, for example, of an alumina ceramic or similar insulating material, and a heating resistor 13 of tantalum nitride (Ta.sub.2 N) formed on the insulating substrate 11. Electric conductors 15, 17, which are, respectively, made of gold and/or chromium, are formed on the heating resistor 13. An oxidation-resistant film 21 made, for example, of silicon carbide (SiC) is provided over the heating resistor 13 and the conductors 15, 17 in order to prevent deterioration of the tantalum nitride heating resistor 13. Over the oxidation-resistant film 21 is formed a wear-resistant layer 23 which can protect the thermal printing head from wearing during printing. The layer 23 is made, for example, of tantalum pentaoxide (Ta.sub.2 O.sub.5), silicon carbide (SiC) or the like. This type of thermal printing head is described, for example, in IEEE, Vol. CHMT-7, No. 3 (September, 1984), p. 294-298. In the thermal printing head of FIG. 5, a portion 19 of the heating resistor 13, which is established between the conductors or electrodes 15 and 17 provided on the heating resistor 13 apart from each other, serves as a heating unit.
In the known thermal printing head having such a structure as described above, the specific resistance of the tantalum nitride (Ta.sub.2 N) is not larger than about 300 micro-ohms.multidot.cm. If the heating resistor 13 is designed to have a thickness sufficient to stand use over a long time period, the resistance of the heating resistor becomes smaller than a desired resistance of, for example, about 10.sup.2 to 10.sup.5 micro-ohms.multidot.cm. Moreover, when any protective layer for the heating resistor is not provided as an oxidation-resistant film or when the thickness of the protective layer is not sufficient, the application of energy necessary for the printing to the heating resistor 13 (hereinafter referred to simply as application energy) will increase the resistance of the heating resistor by the action of the oxidation of the resistor, resulting in deterioration of the heating resistor. This leads to a disadvantage in that the thermal printing head enables one to allow thermal printing only within a very short time period.
On the contrary, when the protective layer is made thicker than as required, the thermal response of the thermal printing head at the time of application or termination of an electric current supplied to the heating resistor remains dull, with an attendant disadvantage in that high speed printing is not possible.
For reproducible thermosensitive transfer printing to which attention has been drawn recently, higher application energy than the energy used in known thermal printing heads is necessary for printing through a film. This requires a larger electric current to be supplied to the heating element so as to ensure satisfactory printing energy.
However, because of the limitations on a circuit and a driving technique for the thermal printing head, a resistor has to be designed of such a form as to give a practical resistance upon application of a limited electric current.
FIGS. 6A and 6B are, respectively, schematic plane views showing heating resistors as used in FIG. 5. In order to effectively operate the heating resistor by application of a limited current, there has now been used a meander-type heating resistor of FIG. 6B, instead of a rectangular heating resistor of FIG. 6A, so that the resistance is increased. However, the heating resistor of the meander form shown in FIG. 6B is more complicated in shape as is seen from the figures. For realization of printing of a higher accuracy, a finer and more accurate shape of the resistor is required, presenting the problem in that a limitation is placed on processing techniques.
Developments of heating resistors have now been made on a variety of compounds used as electrically insulating materials instead of tantalum nitride (Ta.sub.2 N). In one such heating resistor, silicon nitride (SiNx) or silicon oxide (SiOx) is used as an electrically insulating material and tantalum (Ta) is used as a conducting material. However, these materials are not satisfactory with respect to resistances to heat and oxidation.